As materials for fluorescent body, II-VI group semiconductors, such as ZnS, have been studied since a long time ago. However, II-VI group semiconductors have been associated with such a problem that the longevity degrades when they are turned into a highly excited state utilizing electron beam in order to achieve high luminance. Accordingly, the development of highly resistant material has been longed for.
As for highly-resistant fluorescent-body material, it is possible to name nitride semiconductor. Since nitride semiconductor is very stable physically and chemically, no such phenomenon occurs that the longevity degrades even when being turned into an intensely excited state by means of electron beam, and the like, and accordingly is one which can be expected as a highly-resistant fluorescent-body material. However, nitride semiconductor is such that the making of bulk single crystal from molten liquid is difficult. Accordingly, for the making of fluorescent body using nitride semiconductor, the following has been employed conventionally: a method of utilizing the growth of polycrystal powder (see Patent Publication No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 9-235,548, for instance); and a method of utilizing a vapor-phase growth method (see Patent Publication No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 11-339,681, for instance).
In this Patent Literature No. 1, there is disclosed a process for producing granular fluorescent body by means of heating a raw-material powder, which comprises gallium sulfide as a gallium compound not-including oxygen, indium sulfide as an indium compound not-including oxygen, and a sulfide of Zn or Mg as a doping substance not-including oxygen, in an ammonia atmosphere.
On the other hand, in Patent Literature No. 2, there is disclosed a technology for orienting a crystal axis in fluorescent crystal film in which a conductive film is formed on an insulated base plate; and a polycrystal-state fluorescent crystal film comprising InxGayAl1-x-yN: Zn, Si is grown on the conductive film by means of metalorganic gas-phase growth method (or MOCVD method) and thereafter by means of annealing treatment.
In the method that utilizes the powder set forth in aforementioned Patent Literature No. 1, it is possible to obtain nitride semiconductor in a large amount at low cost. However, in the method of heating gallium nitride or gallium oxide, and the like, in an ammonia atmosphere, there is such a problem that the crystal quality is poor, and accordingly it is blackened (or crystals themselves appear to be black) due to nitrogen deficiency so that the rise of luminance becomes difficult. Moreover, since the controllability of raw-material supply is low, and since there are limitations on the raw-material species, there is a limitation on improving the light-emitting efficiency.
On the other hand, in accordance with the method that utilizes the vapor-phase growth method disclosed in aforementioned Patent Literature No. 2, there is no such a problem that the crystal quality degrades due to blackening. Moreover, since the control of raw-material supply is easy, the control of making mixed crystal and the concentration control of impurities become feasible, and thereby the improvement of color rendering property or light-emitting efficiency becomes feasible. However, since a membrane-shaped fluorescent crystal film is formed by means of the vapor-phase growth method, it is difficult to microcrystallize it, and accordingly there is such a problem that it is not possible to improve the light-emitting efficiency extremely.